Dependency Injection

gistfile1.rb

class Animal
  def initialize
    @sound = "Crickets, crickets..."
  end 

  def make_noise
    puts @sound
  end 
end

class Dog < Animal
  def initialize
    @sound = "Woof, woof"
  end 
end

class Cat < Animal
  def initialize
    @sound = "Meow, puurrr"
  end 
end

class Cow < Animal
  def initialize
    @sound = "Moooo"
  end 
end

class PetAnimal
  def initialize(animal = Animal.new)
    @animal = animal
  end 

  def pet 
    @animal.make_noise
  end 
end

PetAnimal.new.pet # "Crickets, crickets..."
PetAnimal.new(Dog.new).pet # "Woof, woof"
PetAnimal.new(Cat.new).pet # "Meow, puurrr"
PetAnimal.new(Cow.new).pet # "Moooo"